Micromachining by chemical mechanical polishing

ABSTRACT

Methods of micromachining structures on substrates using chemical mechanical polishing techniques.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to micromachining techniques. More particularly,the invention relates to methods for micromachining substrates intodefined structures using chemical mechanical polishing.

2. Description of the Related Art

Chemical mechanical polishing (CMP) is used in the integrated circuitindustry for planarizing substrates. In a typical CMP process, thesubstrate is placed in direct contact with a rotating polishing pad. Acarrier applies pressure against the backside of the substrate. Duringthe polishing process, the pad and table are rotated while a downwardforce is maintained against the substrate back. An abrasive andchemically reactive solution, commonly referred to as a “slurry,” isapplied to the pad during polishing. The slurry initiates the polishingprocess by chemically reacting with the film being polished. Thepolishing process is facilitated by the rotational movement of the padrelative to the substrate as slurry is provided to the substrate/padinterface. Polishing is continued in this manner until the desired filmis removed.

A substantial amount of effort is directed in the industry at maximizingthe planarity of substrates being polished by CMP, and minimizing theformation of non-planar features, such as corner rounding, and dishing.As a result, CMP has not been considered by the industry as amicromachining tool.

Current micromachining methods have undesirable attributes. Forinstance, binary optics prepared by current methods typically radiatesubstantial power in undesired orders, and have polarization dependentphase shifts. Consequently, new and improved micromachining methods areneeded for optics fabrication and fabrication of other structures, suchas mechanical devices, imagers, and displays.

SUMMARY OF THE INVENTION

The invention relates to micromachining methods using CMP. Thus, in afirst aspect the invention provides a method for forming a substrateinto a defined structure, said method comprising selectively removing atleast a portion of the substrate by chemical mechanical polishing toprovide the defined structure, wherein the defined structure is at leastpartially non-planar.

The invention also provides structures and devices formed by the methodsdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the formation of a beginning structure using adeformed polishing pad.

FIG. 2 illustrates a polishing sequence that includes an initial step ofhigh pad acceleration followed by subsequent steps of constantacceleration.

FIG. 3 illustrates the polishing of a feature under conditions ofvarying downforce.

FIG. 4 illustrates polishing of a feature by a pad having localcurvature.

FIG. 5 illustrates polishing of a feature by a pad having asperities onits surface.

FIG. 6 illustrates the use of a buffer material in a polishing process.

DESCRIPTION OF THE CURRENT EMBODIMENT

This invention relates to a process for micromachining substrates intodefined structures, using chemical mechanical polishing (CMP).Specifically, CMP is used to form structures that are at least partiallynon-planar. Examples of such partially non-planar structures include,but are not limited to, rounded structures, convex and concave shapes,and combinations thereof, as well as more complex structures. Suchnon-planar structures are generally functional and find use in manyapplications, including photonics, such as microlens arrays and opticalfiber array connectors, mechanical devices, imagers, displays, and soon. In preferred aspects, the height differential between the highestpoint and the lowest point on the surface of non-planar structuresmanufactured according to this invention are 0.5 microns or greater. Inother preferred aspects, the height differential is 1 micron or greater.In further preferred aspects, the height differential is 2 microns orgreater.

CMP is a polishing process that exploits both chemical and mechanicalpolishing techniques for the polishing of a surface. Generally, thesubstrate is placed in direct contact with a polishing pad, and the pad,substrate, or both, are rotated while a downward force is maintainedagainst the substrate back. A slurry, representing the chemical aspectof CMP, is applied to the pad during polishing. Alternatively, anabrasive containing polishing pad may, be used in which case thechemical ingredients of the polishing composition are applied to the padduring polishing.

In the invention, the characteristics of the polishing pad are variedsuch that the CMP polish, including the optional use of a chemicalslurry, provides structures that are at least partially non-planar. Thepolishing pad characteristics that can be varied include the stiffnessof the pad. Pad stiffness can be manipulated by several properties ofthe CMP apparatus, including downforce on the polishing pad; rotationalvelocity of the polishing pad; and acceleration velocity of thepolishing pad. In addition, stiffness is also a function of the padmaterial. Other pad characteristics that are advantageously utilized inthe invention to form partially non-planar structures include areas oflocal curvature on the pad, areas of local increased or decreaseddownforce on the pad, and the presence of asperities on the pad. Inaddition, the position of the polishing pad relative to the substratebeing polished, can be exploited to provide structures that arepartially non-planar.

A substrate to be micromachined according to the invention preferablyincludes one or more beginning features upon which the polishing padacts to effect a change in shape. Such a beginning feature can readilybe prepared by semiconductor techniques known in the art. The choice ofbeginning feature will depend in part on the final geometry that isdesired. One example of a useful beginning feature is a steppedstructure. Alternatively, or in addition, the substrate may include twoor more materials in the same plane that have different properties, suchas different polishing rates, to allow CMP to effect a change in shape.

The characteristics of stiffness of the polishing pad can be manipulatedto provide rounding, such as corner rounding, of a feature. To achievelarge amounts of corner rounding, it is desirable to have a pad that isnot too stiff because it will deform more at the edges of a feature. Thecentrifugal force experienced by the pad at any point on its surfacewill determine the increase in stiffness of the pad (centrifugal forceis controllable by the velocity and/or acceleration of the pad). Sincethe centrifugal force on any point on a disk is proportional to therotational velocity times the distance from its axis of rotation, thestiffness will increase as the distance from the axis increases. Theincrease in stiffness is approximately proportional to the square rootof the increase in centrifugal force.

If a deformable pad, such as one made of a polymer with a significantelastic modulus, and which is not pinned at the back so it is free todeform, rotates at a small angular velocity it will tend not to bestiffened. In such a case, the degree of comer rounding will be great(see FIG. 1, illustrating a beginning structure 10 being polished by adeformed polishing pad 20). If the same pad is rotated at a significantrotational velocity, causing stiffening, the amount of comer roundingwill be less. An adjustment of the relative distance of the featurebeing polished from the rotational axis of the pad can achieve the sameresult as adjusting the rotational speed of the pad because of theeffect noted above. FIG. 2 illustrates a beginning structure 10 that isformed, for example, by a polishing sequence that includes an initialstep of high pad acceleration followed by subsequent steps of constantacceleration.

The deformity of the pad can also be affected by the downforce on thepad. If the relative downforce being applied to the pad is greater itwill tend to bend the pad to a greater degree than if the force beingapplied is of a lesser amount. A stiffer pad will deform less for agiven applied downforce. FIG. 3 illustrates the polishing of a feature10 under conditions of varying downforce. “F=0, F=1” vs. “F=0, F=0.01”refer to the relative values of “Force” being applied by the relativestiffness in the pad and the downforce applied during polishing. Asdiscussed above, the more the pad is deformed, the greater the extent ofcorner rounding.

A further pad characteristic that can be exploited to produce controlledtopography in the polishing surface, is localized non-planarities,bumps, grooves or other surface features on the pad itself. In a typicalpolishing operation, the polishing surface is moved over the surface ofthe pad in a circular motion. Therefore, any surface feature in the padwill affect the material in a circular fashion. A circular ridge orgroove on the pad surface will tend to produce a linear feature on thepolishing surface. A large ridge or large groove on the pad will tend toproduce a corresponding large ridge or groove on the polishing surface.Using intermittent ridges or grooves disposed in a circular manner onpad surface will tend to reduce the rate at which the material was beingremoved but will tend to produce the same feature shape.

One advantage of pad surface features is the flexibility that itprovides to produce a high degree of local curvature in the polishingsurface. The deliberate juxtaposition of an existing surface feature onthe pad and on the polishing surface can be utilized to produce a highdegree of local curvature and counter-curvature (concave and convexcurves on the surface in proximity). A cross-section of a surfacefeature being polished to produce both of these curves is shown in FIG.4.

When material is removed from a step-shaped feature with a pad it ispreferentially removed from the raised comers producing a bevel orconvex shape. As long as the material has a finite removal rate outsidethe step shape, which can be increased through the use of a verycompliant pad, there will be a tendency to produce a counter-curve orconcave shape right beside the convex shape. After the removal of asignificant fraction of the material forming the initial step (probablymore than 50%), the total length of the concave-curved surface willbegin to approach the length of the convex-curved surface.

It is possible to produce this curve and counter-curve without a surfacefeature being present on the pad. However, it is easier to producecurves with a higher degree of curvature with the two sets of featureson the opposing surfaces being in close proximity. The surface featuresin the pad which help to produce the surface features on the polishingsurface should be of the appropriate size to produce the desired featuredimension.

Additional layers of material, such as buffer layers, may be used tounderfill or overfill spaces between features on a substrate, thusfurther enhancing the CMP process. Such layers are described inpublished patent application number U.S. 2003/0136759, which isincorporated herein by reference in its entirety.

When used, a buffer layer can assist in the formation various shapes ona feature, including convex and concave shapes. FIG. 6 a provides oneexample of the use of a buffer material 50 for the formation of concavesurface on structure 10. When used for the formation of a concavesurface on structure 10, the buffer material 50 should be a materialthat is removed by polishing at a rate slower than the rate at whichstructure 10 material is removed during the same polishing procedure.The preferential removal of structure 10 in comparison to buffermaterial 50 causes the formation of a concave surface to structure 10during the polishing step.

Alternatively, buffer material can be used for the formation of aconcave surface on a structure. In this embodiment, the buffer materialshould be a material that is removed by polishing at a rate faster thanthe rate at which the underlying structure material is removed duringthe same polishing procedure. The preferential removal of buffermaterial in comparison to structure material causes the formation of aconvex surface to the structure during the polishing step. The use ofthe two types of buffer materials (those that are polished more quicklyand those that are removed more slowly) on the surface at the same timecan be utilized to produce very complex shapes in polished materials.

A further example of the advantageous use of buffer material for theformation of partially non planar structures is depicted in FIG. 6B. Inthis embodiment, the buffer material acts as a stop layer, protectingany underlying structures from polishing. Thus in this embodiment, thebuffer material 50 should be a material that is removed by polishing ata rate slower than the rate at which structure 10 material is removedduring the same polishing procedure.

In the above embodiments, any buffer material remaining on the substratefollowing polishing can be removed by well known techniques such aschemical etching.

As noted earlier, it is preferred that all polishing steps describedherein be conducted in the presence of a chemical polishing compositionor slurry. The choice of polishing composition or slurry is an importantfactor in the CMP step. Depending on the choice of ingredients such asoxidizing agents, film forming agents, acids, bases, surfactants,complexing agents, abrasives, and other useful additives, the polishingslurry can be tailored to provide effective polishing of the substratelayer(s) at desired polishing rates while minimizing surfaceimperfections, defects and corrosion and erosion. Furthermore, thepolishing composition may be selected to provide controlled polishingselectivities to other thin-film materials used in substratemanufacturing.

Examples of CMP polishing compositions and slurries are disclosed, inU.S. Pat. Nos. 6,068,787, 6,063,306, 6,033,596, 6,039,891, 6,015,506,5,954,997, 5,993,686, 5,783,489, 5,244,523, 5,209,816, 5,340,370,4,789,648, 5,391,258, 5,476,606, 5,527,423, 5,354,490, 5,157,876,5,137,544, 4,956,313, the specifications of each of which areincorporated herein by reference.

While the present invention has been described by means of specificembodiments, it will be understood that modifications may be madewithout departing from the spirit of the invention. The scope of theinvention is not to be considered as limited by the description of theinvention set forth in the specification and examples, but rather asdefined by the following claims.

1. A method for micromachining a structure, said method comprising selectively removing at least a portion of the structure by chemical mechanical polishing, wherein the structure thus formed is at least partially non-planar and wherein the structure includes a highest point and a lowest point, and has a height differential between the highest point and the lowest point of 0.5 microns or greater.
 2. The method of claim 1 wherein the structure is formed on an essentially planar substrate.
 3. The method of claim 1 wherein said chemical mechanical polishing step is conducted using a chemical mechanical polishing apparatus that includes a polishing pad.
 4. The method of claim 3 wherein said removal is by a combination of chemical etch and mechanical polishing.
 5. The method of claim 4 wherein said mechanical polishing is controlled by varying at least one characteristic of the polishing pad.
 6. The method of claim 5 wherein said characteristic of the polishing pad is stiffness.
 7. The method of claim 6 wherein said stiffness is manipulated by downforce on the pad, rotational velocity of the pad, acceleration velocity of the pad, local curvature of the pad, or combinations thereof.
 8. The method of claim 1 wherein a concave structure is formed.
 9. The method of claim 1 wherein a convex structure is formed.
 10. The method of claim 1 wherein a rounded structure is formed.
 11. The method of claim 7 wherein local curvature on the pad is provided be by pre-shaped asperities.
 12. The method of claim 7 wherein local curvature on the pad is provided by bumps under the pad.
 13. (canceled)
 14. The method of claim 1 wherein the height differential between the highest point and the lowest point is 1 micron or greater.
 15. The method of claim 1 wherein the height differential between the highest point and the lowest point is 2 microns or greater.
 16. A partially non-planar structure fabricated by the method of claim
 1. 17. A microlens array fabricated by the method of claim
 1. 18. An optical fiber array connector fabricated by the method of claim
 1. 